1. Field of the Invention
This invention broadly relates to polymeric, alumina filled cast synthetic resin insulators typically used by electrical utilities in connection with their transmission and distribution systems. More particularly, it is concerned with the discovery that certain relatively critical components and proportions thereof can be employed to give a filled synthetic resin insulator having a large number of very desirable properties in the field, while at the same time giving a pourable casting composition which greatly facilitates initial production of the insulator. That is to say, combined in the insulators of the present invention are properties such as excellent arc track and flame resistance, good weatherability, high impact strength without excessive brittleness, and the ability to fabricate the insulators by conventional casting techniques even though the fill loading of the casting compositions is very high, on the order of 60 to 75% by weight.
2. Description of the Prior Art
Elongated, skirted insulators of various types have been in use in the electrical transmission and distribution industry for many years. To be effective, such insulators must be able to withstand ambient weather conditions and be produceable at relatively low cost. The fact that many researchers in the field are still at work in attempting to perfect outdoor high voltage insulators amply attests to the fact that no single insulator construction heretofore available has met all of the criteria needed for long term, trouble-free operation.
For example, porcelain has long been the material of choice for construction of outdoor high voltage insulators. This is because procelain has outstanding weatherability and superior arc track resistance, along with high compressive strength. However, the high weight to strength ratio in brittleness of porcelain insulators is a severe problem from a fabrication handling and use standpoint, resulting in relatively high loss factors. For example, porcelain insulators shatter to a very satisfactory degree when struck by a bullet, and therefore make prime targets for hunters or vandals bent upon destruction. Also, in contaminated areas, porcelain insulators must be coated with a grease-like material that encapsulates dirt particles, in order to keep the dirt from forming a conductive path on the porcelain surface. In order to avoid flashover problems, porcelain insulators must be cleaned and recoated every six months to two years depending upon local conditions--a very costly but necessary operation. As a consequence of the above problems, many utilities have been seeking effective synthetic resin insulators which do not possess the limitations of porcelain and can compete with the long range durability of the latter.
In order to meet this demand, producers have developed a wide variety of synthetic resin based insulators. In general, these have proven to be deficient in one or more important areas such as arc track resistance, weatherability, impact strength or castability.
One class of synthetic resin insulators heretofore available have been formed using relatively high quantities of curable, hydrophobic, thermosetting synthetic resins such as 2,2-bis(p-hydroxyphenyl)propane glycidyl ether polymers (commonly called glicidyl ethers of Bisphenol A type epoxies and referred to herein for convenience as bisphenol A resins or BPA resins). If desired, the resin may be reinforced with a material having insulating properties such as glass fibers. BPA epoxy resins have heretofore been considered to be the most practical base material for porcelain substitutes since they more nearly meet all of the requirements outlined above at a reasonable cost. However, actual commercial practice with BPA epoxy based insulating products in electrical applications has proven that these insulators tend to track, especially after exposure to ambient atmosphere for an extended period. For example, while conventional BPA epoxy resin has excellent properties insofar as dielectric constant, mechanical strength, electrical resistance, and ability to be molded are concerned, such resins when cured with an anhydride and accelerated with an amine accelerator exhibit poor arc track resistance, especially after extended weathering. Thus, a simple unmodified BPA epoxy resin is not the real answer to the problems presented.
It is also known that a conventional BPA epoxy system can be modified to increase the arc track resistance thereof by the addition of a filler thereto such as hydrated alumina. Fillers are advantageous not only because of the increase in arc track resistance obtained thereby, but also by virtue of the fact that use of such fillers correspondingly lowers the amount of synthetic resin material required, thereby lowering production costs. However, such hydrated alumina fillers have the negative effects of increasing the brittleness of the final insulator and making fabrication difficult, especially at high loadings. Specifically, when it is attempted to use filler concentrations of 50% or more by weight, the viscosity of the casting composition increases very rapidly to the point that the composition is not pourable. This problem is particularly acute when smaller particle size alumina is employed. As will be appreciated, lack of easy pourability not only slows the casting process, but can also create voids in the outer surface of the insulator which in turn collect dirt and tend to establish a conductive track across the insulator, and create hidden voids at the interface between the skirt material and the internal glass reinforced epoxy rod. In addition, high filler loadings can be troublesome in that during casting the filler can settle, thus giving a nonuniform composition throughout the insulator.
Because of problems encountered with BPA resin products, cycloaliphatic epoxies have been suggested as an alternate (see U.S. Pat. No. 3,511,922). The cycloaliphatics are of particular interest because, being aliphatic, they tend to burn with a clean flame. The aromatic materials on the other hand burn with a dirty soot-filled flame which tend to form carbon. Thus, under arcing conditions aromatic systems (e.g., BPA epoxies) tend to form a carbon path or track, while the aliphatic systems erode. However, cycloaliphatic systems have been found to be deficient in terms of impact strengths, particularly when filled. Also, cycloaliphatic systems tend to exhibit high erosion rates and flammability.
It has also been suggested to employ aliphatic epoxy resins in connection with electrical insulators (see U.S. Pat. Nos. 3,838,055 and 3,351,574, both of which are incorporated herein by reference). The insulators described in U.S. Pat. No. 3,838,055 preferably include a BPA resin cross linked with a polyglycidyl ether polymer of castor oil. These types of insulators are designed to operate without addition of filler.
Other patents of background interest include: U.S. Pat. Nos. 3,475,546, 3,535,289, 3,551,551, 3,571,491, 3,622,537, 3,645,889, 3,511,922, 3,044,900, 3,567,677, 3,446,741, 3,470,110, 3,445,282, 3,351,574, 3,497,531, 3,251,801 and 3,476,702.